Antigens

Antigens are substances that trigger an immune response in the body. These molecules are often proteins or polysaccharides found on the surface of pathogens like bacteria, viruses, and fungi. When the immune system detects an antigen, it produces antibodies to neutralize or destroy the foreign invader. Antigens can also be toxins or other foreign substances that induce an immune response. They play a crucial role in the body’s ability to recognize and fight off infections, making them essential components of the immune system. Understanding antigens is key to developing vaccines and therapies for various diseases.

What is an Antigen?

An antigen is any substance that provokes an immune response in the body. These substances are usually proteins or polysaccharides found on the surface of pathogens such as bacteria, viruses, or fungi. When the immune system recognizes an antigen as foreign, it produces specific antibodies to target and neutralize the invader. Antigens can also include toxins or other foreign substances that elicit an immune reaction.

Examples of Antigens in Immune Response, Blood, and the Human Body

1. Bacterial Toxins – Substances produced by bacteria, such as the diphtheria toxin, that trigger an immune response.
2. Viral Proteins – Surface proteins of viruses like the spike protein of the SARS-CoV-2 virus, which causes COVID-19.
3. Pollen – Common allergen from plants that can cause allergic reactions in sensitive individuals.
4. Dust Mites – Proteins from dust mite feces that can induce allergic responses.
5. Blood Group Antigens – Surface molecules on red blood cells that determine blood types, such as ABO and Rh factors.
6. Autoantigens – Normal body proteins mistakenly targeted by the immune system in autoimmune diseases, like insulin in type 1 diabetes.
7. Tumor Antigens – Abnormal proteins expressed on cancer cells that the immune system can recognize and attack.
8. Food Proteins – Proteins in foods like peanuts or shellfish that can cause allergic reactions.
9. Vaccines – Inactivated or attenuated pathogens, or parts of pathogens, introduced to stimulate an immune response without causing disease.
10. Parasite Antigens – Proteins on the surface of parasites, such as those from malaria-causing Plasmodium species, that trigger an immune response.

Types of Antigens

Based on Origin

1. Exogenous Antigens
   • Origin: External environment
   • Examples: Bacterial toxins, viruses, pollen
   • Role: Enter the body through inhalation, ingestion, or injection and are processed by antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells.
2. Endogenous Antigens
   • Origin: Inside the body’s cells
   • Examples: Viral proteins produced inside infected cells, abnormal proteins in cancer cells
   • Role: Presented on the cell surface by MHC class I molecules and recognized by cytotoxic T cells.
3. Autoantigens
   • Origin: Normal body tissues
   • Examples: Myelin basic protein in multiple sclerosis, insulin in type 1 diabetes
   • Role: Typically ignored by the immune system, but in autoimmune diseases, they are mistakenly targeted by the immune response.

Based on Function

1. T-independent Antigens
   • Characteristics: Can trigger an immune response without assistance from T-helper cells
   • Examples: Bacterial polysaccharides, lipopolysaccharides
   • Role: Directly stimulate B cells to produce antibodies.
2. T-dependent Antigens
   • Characteristics: Require T-helper cells to provoke an immune response
   • Examples: Proteins from pathogens, vaccines
   • Role: B cells process and present these antigens to T-helper cells, which then aid in B cell activation and antibody production.

Based on Immunogenicity

1. Complete Antigens
   • Characteristics: Can induce a full immune response on their own
   • Examples: Whole pathogens like bacteria, viruses
   • Role: Possess both immunogenicity (ability to provoke an immune response) and antigenicity (ability to specifically bind to antibodies or immune cells).
2. Incomplete Antigens (Haptens)
   • Characteristics: Cannot induce an immune response by themselves
   • Examples: Small molecules like penicillin, urushiol (poison ivy toxin)
   • Role: Become immunogenic only when attached to larger carrier molecules, forming a complete antigen.

Antigen-Presenting Cells (APCs)

Antigen-presenting cells (APCs) are a crucial component of the immune system. They capture, process, and present antigens to T cells, initiating an immune response. APCs display fragments of antigens on their surface using molecules called major histocompatibility complex (MHC) proteins. This presentation allows T cells to recognize and respond to the antigens, either by activating other immune cells or by directly attacking infected cells.

Types of Antigen-Presenting Cells

1. Dendritic Cells
   • Location: Found in tissues that are in contact with the external environment, such as the skin (Langerhans cells), and the inner lining of the nose, lungs, stomach, and intestines.
   • Function: Capture antigens, migrate to lymph nodes, and present them to T cells. They are the most effective APCs and are essential for initiating T cell responses.
2. Macrophages
   • Location: Present in almost all tissues, particularly abundant in the liver (Kupffer cells), lungs (alveolar macrophages), and lymph nodes.
   • Function: Engulf and digest pathogens and debris. After processing antigens, they present them to T cells, especially during the later stages of an immune response.
3. B Cells
   • Location: Found in the blood, lymph nodes, and spleen.
   • Function: Bind specific antigens through their B cell receptors (BCR), process the antigens, and present them to helper T cells. This interaction is crucial for B cell activation and subsequent antibody production.

Antigen Functions

1. Immune System Activation

Antigens trigger the activation of the immune system. When antigens are detected, the body responds by producing antibodies and activating various immune cells to combat the invading pathogens.

2. Specificity

Antigens determine the specificity of the immune response. Each antigen has a unique molecular structure that is recognized by specific antibodies or immune cells, ensuring that the immune response targets the correct pathogen.

3. Memory Formation

Antigens are essential for the formation of immunological memory. Once the immune system encounters an antigen, it “remembers” it, leading to a quicker and more efficient response if the same antigen is encountered again in the future.

4. Pathogen Identification

Antigens help the immune system identify and differentiate between self and non-self molecules. This is crucial in distinguishing between the body’s own cells and harmful pathogens like bacteria, viruses, and fungi.

5. Vaccination Response

Vaccines contain antigens that mimic specific pathogens. When administered, these antigens stimulate the immune system to produce a protective response without causing disease, thereby providing immunity against future infections by the actual pathogen.

6. Activation of T-Cells

Antigens are presented by antigen-presenting cells (APCs) to T-cells, which are critical components of the adaptive immune system. This interaction leads to the activation and proliferation of T-cells, which then assist in eliminating the pathogen.

7. B-Cell Activation

Antigens bind to B-cell receptors on the surface of B-cells. This binding, along with signals from helper T-cells, activates B-cells to proliferate and differentiate into plasma cells that produce antibodies specific to the antigen.

8. Allergy Development

In some cases, antigens can trigger allergic reactions. These antigens, known as allergens, cause the immune system to overreact, leading to symptoms like inflammation, itching, and respiratory issues.

9. Autoimmune Responses

In autoimmune diseases, the immune system mistakenly targets self-antigens, which are the body’s own molecules. This leads to the immune system attacking healthy tissues, causing conditions like rheumatoid arthritis and lupus.

Function	Description
Immune System Activation	Triggers the immune system to produce antibodies and activate immune cells.
Specificity	Ensures the immune response targets the correct pathogen.
Memory Formation	Enables quicker and more efficient responses to previously encountered antigens.
Pathogen Identification	Helps distinguish between self and non-self molecules.
Vaccination Response	Mimics pathogens to provide immunity without causing disease.
Activation of T-Cells	Leads to the activation and proliferation of T-cells.
B-Cell Activation	Stimulates B-cells to produce specific antibodies.
Allergy Development	Can cause allergic reactions by triggering an overactive immune response.
Autoimmune Responses	Results in the immune system attacking the body’s own tissues.

Where Are Antigens Found?

Antigens are substances that trigger an immune response in the body. They can be found in various locations and forms, including:

1. On the Surface of Pathogens

• Bacteria: Antigens are present on the cell walls, membranes, and flagella.
• Viruses: Viral antigens are found on the protein coat or envelope surrounding the viral genome.

2. On Infected Cells

• Virus-Infected Cells: When a virus infects a cell, viral proteins (antigens) are displayed on the cell’s surface.
• Bacteria-Infected Cells: Similarly, cells infected by bacteria may present bacterial antigens on their surfaces.

3. In Vaccines

• Live Attenuated Vaccines: Contain weakened pathogens with surface antigens.
• Inactivated Vaccines: Contain killed pathogens or specific parts of pathogens (like proteins) that act as antigens.
• Subunit Vaccines: Include only the antigens that best stimulate the immune system.

4. On Transplanted Organs and Tissues

• Donor Tissues: Organs or tissues from donors have antigens that may be recognized as foreign by the recipient’s immune system, potentially leading to rejection.

5. In Blood and Body Fluids

• Blood Types: Blood group antigens are found on the surface of red blood cells (A, B, AB, and O antigens).
• Allergens: Certain proteins in food, pollen, and other substances can act as antigens, causing allergic reactions.

6. On Cancer Cells

• Tumor Antigens: Cancer cells often express unique antigens or overexpress normal proteins, which can be recognized by the immune system.

How Do Antigens Enter the Body?

Antigens are substances that can trigger an immune response. They are typically foreign molecules such as bacteria, viruses, fungi, and toxins. Understanding how antigens enter the body is crucial for comprehending how our immune system detects and responds to potential threats. Here are the primary ways antigens can enter the body:

1. Respiratory Tract

The respiratory tract is one of the most common entry points for antigens. Airborne pathogens like bacteria, viruses, and allergens can enter the body through inhalation. Once inside, these antigens can cause respiratory infections and trigger immune responses.

Examples:

• Viruses: Influenza, COVID-19
• Bacteria: Streptococcus pneumoniae, Mycobacterium tuberculosis
• Allergens: Pollen, dust mites

2. Gastrointestinal Tract

The gastrointestinal (GI) tract allows antigens to enter the body through ingestion. Contaminated food and water can introduce bacteria, viruses, and parasites into the digestive system.

Examples:

• Bacteria: Salmonella, Escherichia coli
• Viruses: Norovirus, Hepatitis A
• Parasites: Giardia, Entamoeba histolytica

3. Skin

The skin serves as a physical barrier, but antigens can still enter through cuts, abrasions, or insect bites. Direct contact with contaminated surfaces or objects can also introduce pathogens through the skin.

Examples:

• Bacteria: Staphylococcus aureus (through cuts)
• Viruses: Rabies virus (through animal bites)
• Parasites: Plasmodium (through mosquito bites)

4. Urogenital Tract

Sexually transmitted infections (STIs) are common ways for antigens to enter the body through the urogenital tract. Pathogens can be transmitted during sexual contact.

Examples:

• Bacteria: Neisseria gonorrhoeae, Chlamydia trachomatis
• Viruses: Human Immunodeficiency Virus (HIV), Herpes Simplex Virus (HSV)

5. Bloodstream

Antigens can directly enter the bloodstream through intravenous routes. This can occur via contaminated needles, transfusions, or insect vectors.

Examples:

• Viruses: Hepatitis B and C (through contaminated needles)
• Bacteria: Borrelia burgdorferi (through tick bites causing Lyme disease)
• Parasites: Trypanosoma (through insect vectors causing Chagas disease)

6. Mucous Membranes

Mucous membranes line various cavities in the body and can serve as entry points for antigens. These include the eyes, nose, mouth, and genitals.

Examples:

• Viruses: Adenovirus (through the eyes), Human Papillomavirus (HPV) (through the genitals)
• Bacteria: Neisseria meningitidis (through the nose)

What happens When an Antigen Enters Your Body?

When an antigen enters your body, the immune system quickly detects it as a foreign invader. Antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells engulf and process the antigen, displaying its fragments on their surface bound to major histocompatibility complex (MHC) molecules. This presentation activates helper T cells, which in turn stimulate B cells to produce specific antibodies and cytotoxic T cells to attack infected cells. The antibodies neutralize or mark the antigen for destruction, while cytotoxic T cells directly kill infected cells. This coordinated response aims to eliminate the antigen and protect the body from harm.

What is the Strongest Antigen?

The strongest antigen is generally considered to be the protein antigen, due to its complex and diverse structure, which can elicit a robust immune response. Proteins are made up of long chains of amino acids folded into specific shapes, providing numerous unique sites for immune cells to recognize and bind. This makes protein antigens highly effective at triggering both the humoral (antibody-mediated) and cellular immune responses. Examples of strong protein antigens include those found on the surfaces of pathogens like the hemagglutinin protein of the influenza virus and the spike protein of the SARS-CoV-2 virus, both of which are highly immunogenic and essential targets for vaccines.

Self Antigen

Self-antigens are normal proteins or molecules present within the body that the immune system typically recognizes as “self” and does not attack. These antigens are crucial for maintaining immune tolerance, preventing the immune system from targeting the body’s own tissues. In autoimmune diseases, this tolerance breaks down, leading the immune system to mistakenly identify self-antigens as foreign. This results in an immune response against the body’s own cells and tissues, causing inflammation and damage. Examples of self-antigens involved in autoimmune diseases include myelin basic protein in multiple sclerosis and insulin in type 1 diabetes. Understanding self-antigens is essential for developing treatments that restore immune tolerance and prevent autoimmunity.

Properties of Antigens

1. Immunogenicity

Immunogenicity refers to the ability of an antigen to provoke an immune response. An effective antigen must be recognized as foreign by the immune system and must be able to stimulate the production of specific antibodies or activate specific immune cells.

2. Antigenicity

Antigenicity is the capacity of an antigen to bind specifically to the products of the immune response, such as antibodies or T-cell receptors. An antigen’s antigenicity determines how well it can be identified and targeted by the immune system.

3. Molecular Size

Larger molecules are generally more immunogenic than smaller ones. Typically, molecules with a molecular weight of over 10,000 daltons are considered good antigens, as their size and complexity provide multiple epitopes for immune recognition.

4. Chemical Complexity

Antigens with a more complex chemical structure tend to be more immunogenic. Proteins, with their diverse amino acid sequences and complex three-dimensional structures, are usually stronger antigens compared to simpler molecules like lipids or polysaccharides.

5. Foreignness

The degree of foreignness to the host organism influences an antigen’s immunogenicity. The more different an antigen is from the host’s own molecules, the stronger the immune response it will elicit. This is because the immune system has evolved to distinguish self from non-self molecules.

6. Epitope Density

Epitopes are specific parts of an antigen that are recognized by the immune system. Antigens with a high density of epitopes are more likely to be immunogenic because they provide multiple binding sites for antibodies or T-cell receptors.

7. Degradability

Antigens that can be easily processed and presented by antigen-presenting cells (APCs) are more likely to elicit an immune response. Degradability allows antigens to be broken down into peptides that can be displayed on the surface of APCs for recognition by T-cells.

8. Route of Entry

The route by which an antigen enters the body can affect its immunogenicity. Different routes, such as intramuscular, subcutaneous, oral, or respiratory, can influence the type and magnitude of the immune response.

9. Dose

The amount of antigen introduced into the body can impact the immune response. Both very low and very high doses of an antigen may result in weak immune responses, whereas an optimal intermediate dose tends to induce a stronger response.